The general hypotheses to be tested are: 1) NO and RSNOs/OONO- (exemplary protein thiol NO+donating and oxidizing agents, respectively) can modulate L-type calcium current (IcaL) by two distinct mechanisms: i) an indirect mechanism involving activation of soluble guanylyl cyclase (NO); and ii) a direct mechanism involving either S-nitrosylation (RSNOs) or oxidation (RSNOs/OONO-) of critical thiol groups in sarcolemmal and/or extracellularly accessible "redox switches" located either in the L-channel subunit complex or an associated regulatory molecule(s); 2) Specific cellular mechanisms underlying these effects may vary significantly among different cardiac myocyte types (working versus pacemaking myocytes); and 3) Cellular redox state of critical thiols located in the sarcolemmal/ extracellular redox switches is an important determinant in both modulation of cardiac ICaL and in the response "NO". These issues will be addressed using a combination of patch clamp techniques (both whole cell and single channel) to study native IcaL in ferret enzymatically isolated myocytes (ventricle, SA node) and molecular biological techniques (immunolocalization, in situ hybridization) to determine the tissue, cellular and subcellular correlates underlying the functional effects of NO- and redox-related modulation of IcaL.